1. Field of the Invention
This invention relates generally to catheter-tip pressure transducers, and more particularly to methods and apparatus for "zeroing" and calibrating medical catheter-tip gauge-pressure transducers.
2. Prior Art
Catheter-tip pressure transducers are sometimes used, because of the dynamic error encountered in external measurement of intravascular pressure through a long narrow catheter. (By "dynamic error" I refer to poor fidelity in the tracking of rapid pressure variations--such as are found in cardiovascular pressure measurements.)
Heretofore, however, surgical workers and researchers alike have known no effective technique or apparatus for "zeroing" and calibrating ordinary catheter-tip pressure transducers in vivo--that is, while the transducer is in place within a patient's body. (By "calibrating" I mean adjusting the calibration factor or "sensitivity" of an instrumentation package that includes a pressure transducer and a display apparatus.)
In the medical field the usual technique of zeroing and calibrating a pressure transducer involves substituting a zero pressure, and then a known reference pressure, at the measurement side of the transducer, and adjusting the display apparatus to read as it should for the zero and known reference pressures respectively. Zeroing is performed before calibrating, since the calibration process then requires only matching of the readout at one point, near full-scale; and since interactions of the zero and readout mechanisms are minimized by proceeding in this order.
Such techniques normally are impermissible in vivo, because the measurement side is applied to the patient's blood stream or is within some other conduit or vessel of the body. Changing the pressure within the patient's body is not practical for ordinary catheter usage.
The words "normally" and "ordinary" are used in the preceding paragraph because one system has been put forward that is intended to permit in vivo calibration by, in effect, changing the pressure on the measurement side of the transducer. This system, disclosed in U.S. Pat. No. 3,703,099 to William Rouse, involves a complicated special catheter structure that is not actually available.
Zeroing and calibration techniques immediately prior to catheterization are generally impractical due to instrument-sterilization requirements--and in any case are inadequate for the duration of application of the pressure-measurement system.
Consequently, medical personnel have been limited to checking the transducer zero and calibration after completion of the particular procedure calling for catheterization. Such a sequence of events has several drawbacks. First, medical judgments and even irreversible decisions are sometimes rendered on the basis of the pressure indications made during the particular procedure.
Moreover, even judgments rendered after the medical procedure and the zeroing and calibration are all complete may be misleading. There is a possibility of significant drift in zero or calibration, or both, between the time of the intravascular measurements and the time of subsequent zeroing and calibration. Cather-tip pressure transducers are commonly expected to exhibit significant drift of both zero and sensitivity with time, temperature and use.
Furthermore, catheter pressure transducers commonly exhibit nonlinearity and hysteresis. When these exceed acceptable levels, as sometimes happens, post-measurement zeroing and calibration are inadequate to rescue the intravascular measurements or any decisions made in reliance upon them.
Checking the nonlinearity and hysteresis, however, in effect requires making multiple measurements of calibration. As already explained, in the prior art even one such measurement has been impractical in vivo, except by the employment of a complicated, unavailable special catheter structure.
It will be helpful to review briefly the configuration of equipment used in the prior art. Typically the electrical leads of a catheter-tip transducer either run through a lumen of the catheter, or are embedded in a solid part of the catheter.
It is gauge pressure that is measured with catheter-tip transducers; consequently the catheter has a lumen that conducts atmospheric pressure to the reference side of the transducer--i.e., to the "back" of the transducer diaphragm. This lumen may be used to carry the electrical leads too (if they run through a lumen), or this lumen may be one that is provided in addition to any lumen used for the transducer leads. To avoid contamination the opening of this lumen that is connected to the reference side of the transducer is typically buried inside the connector (with free access to atmospheric pressure) of the electrical cable that carries the transducer excitation and signals between the catheter and the instrumentation.
(In this field of catheters the terms "proximal" and "distal" are customarily referred to the operator, rather than to the patient's body. Hence the open end of the atmospheric-pressure lumen that is outside the patient's body and most proximal to the operator and the electronics is known as the "proximal" end. The same convention is used for other features of the catheter.)
Such a catheter often has at least one other lumen, which in this document will be called an "auxiliary" lumen. Such an auxiliary lumen is used in the prior art to administer medication or withdraw blood samples at or near the pressure-measurement site.
Based upon the foregoing discussion it may be understood that prior art in the field of catheter-tip transducers fails to provide reliable and safe measurement of cardiovascular and other rapidly varying pressures using ordinary catheter-tip transducers.